Although electrical phenomena in the earth's atmosphere have been studied since the eighteenth century, the realization of the existence of a static field having a vertical potential gradient only occurred approximately fifty years ago. This static electric field was determined to exhibit a change in potential which decreases with increasing altitude. At an altitude of approximately 50 kilometers, the atmosphere effectively acts as a perfect conductor. This conducting layer of the atmosphere (known as the electrosphere) together with the earth forms a spherical condenser, the potential of the electrosphere is about 300,000 volts positive relative to the earth (which is conventionally taken to have a potential of zero). Therefore, the electric field which exists in the atmosphere would be considered to be negative (i.e., "pointed" toward the earth) according to "natural" co-ordinate systems. However, this field is usually denoted as "positive" in the study of atmosphere electricity. This sign confusion can be avoided by referring to the atmospheric electric field as the "potential gradient", the graident thus being conventionally "positive" for normal atmospheric conditions.
Hill, in U.S. Pat. No. 3,868,074 inter alia, showed that the static electric field in the atmosphere could be utilized to obtain a reference signal which would be employed inter alia to control both pitch and roll axes stabilization of airborne vehicles. In a more general sense, Hill showed that lines and planes of equipotential existed in the atmosphere, these lines and planes being generally parallel to the surface of the earth absent any disturbing conditions. Hill further provided methods and means for defining such equipotential lines and planes, the lines and planes so defined being useful as a reference for aircraft stabilization and gyroscopic drift correction inter alia.
The present invention provides further definition of the utility of this static electric field existing in the atmosphere. In particular, the invention provides a method and apparatus for detecting and utilizing spatial disturbances in the atmosphere electric field, thereby to enable the avoidance of both man-made and natural orographic protrusions extending from the nominal surface of the earth. Since the conductivity of the earth, or of any orographic protrusion extending into the atmosphere therefrom, is so much greater than that of the atmosphere, the boundary condition for the Poissons equation describing the atmospheric potential at the earth's surface and at the surface of any protrusion is a constant conventionally taken as zero. Thus, at the surface of the earth and at the surfaces of natural and man-made protrusions extending from the earth, an equipotential surface or surfaces exist and the atmospheric field in the vicinity thereof is distorted. In a practical sense, lines and planes of equipotential in the vicinity of orographic protrusions are not parallel to the nominal surface of the earth. Therefore, in a practical sense, the present invention hereby provides teachings which enable the detection and utilization of these spatial disturbances for terrain avoidance. Terrain obstacles of both natural and man-made origin, which can be generically classed as orographic protrusions, perturb the electric field to the degree that the normally vertical potential gradient has both a vertical and a horizontal component. The unperturbed electric field always has a vertical component, the horizontal component of the potential gradient being essentially zero.
According to the present invention, an airborne vehicle in flight over terrain from which significant orographic protrusions extend, can avoid such protrusions by detection of perturbations in the potential gradient which are caused by the protrusions. If the vehicle is on a horizontal flight path lying below or near the altitude of the protrusion, perturbations in the potential gradient can be detected which can usually be used to avoid the protrusion. In a situation where a protrusion is essentially vertical, such as a radio tower or the like, the perturbations usually do not extend sufficiently far from the protrusion to allow detection in time for conventional aircraft to avoid a potential collision. Nevertheless, the principles are useful for slow moving vehicles such as helicopters. The perturbations due to the typical "hill" or mountain ridge can be detected at distances of five times the height of the hill or mountain ridge. Orographic protrusions of this type are of primary interest in terrain avoidance, can be avoided by aircraft through measurement of the horizontal component of the perturbed electric field, the horizontal component of the field being seen to increase as the airborne vehicle approaches the protrusion.
The present invention particularly provides a method wherein the atmospheric potential is sensed at two spaced points in the atmosphere on or near an airborne object, such as an airplane, operating in the atmosphere. Subject to disposition of the potential sensing apparatus and corrections for positioning thereof on the airborne object, the measured potentials will be equal if the object maintains a "level" flight attitude according to the teachings of Hill in the aforementioned U.S. Pat. No. 3,868,074. In said patent, the existence of horizontal lines and planes of equipotential parallel to the earth's nominally flat surface is shown, the potentials measured at two points lying effectively in the same plane being equal. However, according to the present teachings, it is seen that orographic protrusions distort the static electric field to cause said field to generally follow the contours of the protrusion. Thus, as long as the horizontal altitude of the airborne object is maintained, a difference in potential measured between two points as aforesaid indicates the existence of a distorted static electric field. Such a distorted electric field, absent weather-related distortions, indicates the presence of an orographic protrusion. Apparatus embodying the invention can be configured to provide an indication of the presence of the protrusion on a meter or the like or can include servomechanical apparatus which automatically causes the airborne object to avoid the protrusion.
The preferred apparatus of the invention comprises potential sensing probes (which probes may be comprised of radioactive material in order to increase the electrical contact thereof with the air) and a differential voltage amplifier. The amplifier receives the sensed potentials from the probes and detects any difference in the sensed potentials to produce a potential difference signal which may be read out on a gauge or utilized to control a servomechanical or other apparatus. The combination of the potential sensing probes and the differential static amplifier is referred to hereinafter as a differential static voltmeter. In combination with an associated device for providing a stable horizontal reference, perturbations or distortions in the static electric field indicative of orographic protrusions can be identified. This associated device can take the form of a vertical reference gyroscope which defines an artificial horizontal reference with which the airborne object can be aligned.
Thus, it is an object of the invention to provide a method and apparatus for avoiding orographic protrusions extending from the earth's surface.
It is another object of the invention to detect potential differences between at least two spaced points on an airborne vehicle flying substantially level relative to an artificial horizontal reference, a detected potential difference indicating the presence and location of an orographic protrusion.
It is a further object of the invention to provide a differential voltmeter in combination with a vertical reference gyroscope for detecting terrain features extending from the earth's surface.
Further objects and advantages will become more readily apparent in light of the following detailed description of the invention.